My premium membership will end at 22 April 2016, why can´t to keep my house until this date?

[ChantillyTex]

Did you abandon your Linden house so that you could cancel your premium account?  We are all residents here, so can't offer an explanation of why that is necessary immediately when your paid-up premium account still has a way to go.  Presumably, it's cleaner and less hassle for LL to make sure you've met all the qualifications up front for returning to basic account status, at the time you make that decision.  

In hindsight, if you wanted to keep using your house awhile longer, it migiht have been best to hold off on cancelling the premium access until closer to the renewal date.   

[ChantillyTex]

Did you abandon your Linden house so that you could cancel your premium account?  We are all residents here, so can't offer an explanation of why that is necessary immediately when your paid-up premium account still has a way to go.  Presumably, it's cleaner and less hassle for LL to make sure you've met all the qualifications up front for returning to basic account status, at the time you make that decision.  

In hindsight, if you wanted to keep using your house awhile longer, it migiht have been best to hold off on cancelling the premium access until closer to the renewal date.   

[Lindal Kidd]

Chantilly is correct.

When you cancel a Premium subscription, first you have to abandon your Linden Home (and also abandon or sell any other Mainland you own, and remove any tier donations to groups.)  This is because non-Premium members can't own Mainland.

Then, when you cancel the membership, you don't get a refund for unused time.  Instead, your account will show "cancellation pending" until the end of your current subscription period.  You will continue to receive all Premium benefits (except land ownership) until the subscription ends.

If you want to continue to use a Linden Home, you can cancel your downgrade, and then get another Linden Home.  Then, when you get to within a few days of April 22, you can cancel your subscription again, if you decide you still want to.